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Dr. Fazlul Islam
First Edition: March, 2010

Copyright © 2010 reserved by the author

Published by:
Hazrat Shahjalal (Ra.) Foundation
Sylhet, Bangladesh.

Cover Design:
Md. Anwar Hussain
409, Forid Plaza, Zindabazar,
Sylhet, Bangladesh.






Modern Astronomy


Theoretical Astronomy


Observational Astronomy


Observation of the Solar System


Observation of the Sun


Observation of the Earth


Observation of the Stars


Observation of the Pulsars


Observation of the Galaxies


Observation of the Quasars


Physical Cosmology




 Religion and Spirituality


 World Religions


 Timeline of Religion


Messengers of God


Hindu Cosmology


Buddhist Cosmology


Biblical Cosmology



Islamic Cosmology


Spiritual Cosmology


Timeline of Cosmology


Ideas of Heaven and Hell in Hinduism


Concepts of Heaven and Hell in Buddhism


The Hebrew Visions of Hell and Paradise


The Heaven and Hell in Christianity


The Hell and Paradise in Islam




History of Astronomy


Space Exploration


Discovery of Hell


The Hell Sighting Effect


 The Green House Effect


Existence of God


Second Coming of Jesus


The Universal Religion


Cosmic Therapy









2009 was declared by the UN to be the international year of astronomy 2009.
The focus was on enhancing the public’s engagement with and understanding of
astronomy. The International Astronomical Union (IAU) launched the year
under the theme ‘The Universe, Yours to Discover’.
By scientific exploration of the universe, the astronomers have discovered many
celestial bodies in the outer space, which were hidden and unknown to mankind
in the ancient periods. The hell and paradise are two hidden or spiritual matters
of the universe proposed by the prophets. For long time their existence in the
universe was a religious belief. Now, by comparative study of astronomical
discoveries with the religious cosmology, the distant celestial bodies the stars,
galaxies, quasars, pulsars, and black holes all are identified as the hells of
religious cosmology. The location and physical characteristics of all the celestial
bodies are almost identical to those of the spiritual hells that had been told in the
Holy Scriptures.
In the Holy Scriptures of Judaism, Christianity and Islam, it had been stated that
at the day of final judgment the earth would be changed to a big field known as
the field of resurrection, and it would be surrounded by the hells. A very long
bridge will be built over the hells extending from the earth to the paradise. After
the judgment believers will pass over the bridge easily and safely. The
unbelievers will fall in to the hells. The Hindus and Buddhists compare the Hell
to a river of fire — Vaitarani, which will have to pass to reach the gardens of
In the 16th century, Italian astronomer Giordano Bruno proposed that the stars
were actually other suns like our own. In 1838, German astronomer Friedrich
Bessel measured the parallax of 61 Cygni star as 0.314 arc seconds, which given
the diameter of the earth’s orbit indicated that the star was about 9.8 light years
away that had proved the stars to be far remote from the earth’s atmosphere and
all were suns.


In the 20th century with the advent of improved technology, it was proven that
other stars were similar to our own sun, and the sun was found to be part of a
galaxy made up of more than 10 billion stars. Astronomers estimate that there
are at least 70 sextillion (7×1022) suns or stars in the observable universe. Stars
are normally grouped into galaxies. A typical galaxy contains hundreds of
billions of stars, and there are about 125 billion (1.25x1011) galaxies in the
observable universe.
Most galaxies are 1000 to 100,000 parsecs in diameter and are usually separated
by distances on the order of millions of parsecs or mega parsecs. The majority
of galaxies are organized into a hierarchy of associations called clusters, which
in turn can form larger groups called super clusters. These larger structures are
generally arranged into sheets and filaments, which surround immense voids in
the universe.
The Jewish Rabbis teach that there is an upper and a lower paradise containing
apartments for the residence and reward of righteous. They believe that there is
an upper and a lower hell. In the hell there are seven divisions, in each division
six thousand house and in each house six thousand chests, and in each chest six
thousand barrels of gall. Each of the division of the hell to be as depth as one
can walk in three hundred years.
According to religious cosmology, the earth is surrounded by the hells. The
Ulama and Islamic scholars have been recognizing the Sun as a hell since the
seventh century. Today astronomers confirm that all the stars in the galaxies are
suns and all the suns are located around the earth. So the Sun and Sun like
billions of stars in the galaxies, the quasars, pulsars, and black holes, which are
situated around the earth in the regions of hells, all are hells.
Thousands of prophets all over the world told their nations about the existence
of hell and paradise in the universe. For long time in scientific point of view, the
hell and paradise were two religious beliefs proposed by the prophets. By
scientific study of the universe, the existence of spiritual hell now has been


discovered in the outer space that makes the religious belief a scientific fact.
The spiritual hell is now visible in the outer space by thousands of telescopes
from all around the world. Astronomers confirm the message of the prophets
that the hell is awaiting in the outer space for the punishment of unbelievers.
Astronomical discovery of spiritual hell in the universe that also suggests the
existence of spiritual paradise in the deeper and distant regions of the universe
will activate mankind for a normal religious life style and behaviour to remain
safe from the hell fire and enter into the eternal paradise. This discovery will
also unite all the nations of the world for an effective and integrated effort for
salvation from the punishment of hell fire and enjoyment of a happier life in the
paradise forever that might restore peace and discipline all over the world.
The Holy Scriptures predicted two resurrections in the world. The first
resurrection is due to appearance of a fire in the sky and the second, after ending
life on the earth. The first resurrection, which is now imminent all over the
world for astronomical discovery of spiritual hell in the universe, purifying
mankind and sterilizing the world off sins and evil deeds will save billions of
people from the hell fire, open the doors of eternal paradise, prevent greenhouse
effect and ozone depletion, prolong life on the earth, and change our sick world
to a pious, happier, healthier, and prosperous world like a paradise.
I have no ability at all to expose any mystery of the universe and change our
sick world to a prosperous world by a spiritual renewal, and I claim no
originality for what has been compiled from the materials collected from
different scientific and religious authorities whose works I have freely consulted
and utilized. My grateful thanks are due to them.
I hope this discovery will be accepted by the astronomers, scientists, religious
specialists and peoples of all walks of life as the most wonderful and biggest
scientific discovery in the history of mankind for “Hell Sighting Effect” all over
the world.


Astronomy is the scientific study of celestial objects and phenomena that
originate outside the Earth’s atmosphere. It is concerned with the evolution,
physics, chemistry, meteorology, and motion of celestial objects, such as stars,
planets, comets, and galaxies, as well as the formation and development of the
universe. Astronomy is one of the oldest sciences. Astronomers of early
civilizations performed methodical observations of the night sky, and
astronomical artifacts have been found from much earlier periods. However, the
invention of the telescope was required before the astronomy was able to
develop into a modern science. Today, thousands of astronomers are observing
the universe using sophisticated ground-based and orbiting telescopes from all
around the world.1, 2
Astronomy is the oldest of the natural sciences, dating back to antiquity, with its
origins in the religious, mythological, and astrological practices of pre-history.
Early astronomy involved observing the regular patterns of the motions of
visible celestial objects. In some cultures astronomical data was used for
astrological prognostication. Astronomy is not to be confused with astrology,
which claims that human affairs are correlated with the positions of celestial
objects. Although the two fields share a common origin and a part of their
methods (namely, the use of ephemerides), they are distinct.3
Ancient astronomers were able to differentiate between stars and planets, as
stars remain relatively fixed over the centuries while planets will move an
appreciable amount during a comparatively short time. Historically, astronomy
has included disciplines as diverse as astrometry, celestial navigation,
observational astronomy, the making of calendars, and even astrology, but
professional astronomy is nowadays often considered to be synonymous with
astrophysics. Astrophysics is the study of physics of the universe, including the
physical properties of astronomical objects.4


Since the 20th century, the field of professional astronomy split into
observational and theoretical branches. Observational astronomy is focused on
acquiring and analyzing data, mainly using basic principles of physics.
Theoretical astronomy is oriented towards the development of computer or
analytical models to describe astronomical objects and phenomena. The two
fields complement each other, with theoretical astronomy seeking to explain the
observational results, and observations being used to confirm theoretical results.

At the end of the 19th century it was discovered that, when decomposing the
light from the Sun, a multitude of spectral lines were observed (regions where
there was less or no light). Experiments with hot gases showed that the same
lines could be observed in the spectra of gases, specific lines corresponding to
unique elements. It was proved that the chemical elements found in the Sun
were also found on Earth. During the 20th century spectrometry (the study of
these lines) advanced, especially because of the advent of quantum physics that
was necessary to understand the observations.5
Most of our current knowledge of celestial objects was gained during the 20th
century. With the help of the use of photography, fainter objects were observed.
Our sun was found to be part of a galaxy made up of more than 1010 stars (10
billion stars). The existence of other galaxies, one of the matters of the great
debate, was settled by Edwin Hubble, who identified the Andromeda nebula as
a different galaxy, and many others at large distances and receding, moving
away from our galaxy.
Physical cosmology, a discipline that has a large intersection with astronomy,
made huge advances during the 20th century, with the model of the hot big bang
heavily supported by the evidence provided by astronomy and physics, such as
the redshifts of very distant galaxies and radio sources, the cosmic microwave
background radiation, Hubble’s law and cosmological abundances of elements.6


Late in the 19th century, scientists began discovering forms of light which were
invisible to the naked eye: X-Rays, gamma rays, radio waves, microwaves,
ultraviolet radiation, and infrared radiation. This had a major impact on
astronomy, spawning the fields of infrared astronomy, radio astronomy, x-ray
astronomy and finally gamma-ray astronomy. With the advent of spectroscopy
it was proven that other stars were similar to our own sun, but with a range of
temperatures, masses and sizes. The existence of our galaxy, the Milky Way, as
a separate group of stars was only proven in the 20th century, along with the
existence of external galaxies, and soon after, the expansion of the universe seen
in the recession of most galaxies from us.7, 8, 9

Solar Astronomy
At a distance of about eight light-minutes, the most frequently studied star is the
Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr
in age. The Sun is not considered a variable star, but it does undergo periodic
changes in activity known as the sunspot cycle. This is an 11-year fluctuation in
sunspot numbers. Sunspots are regions of lower-than-average temperatures that
are associated with intense magnetic activity.
The Sun has steadily increased in luminosity over the course of its life,
increasing by 40% since it first became a main-sequence star. The Sun has also
undergone periodic changes in luminosity that can have a significant impact on
the Earth. The Maunder minimum, for example, is believed to have caused the
Little Ice Age phenomenon during the Middle Ages.10
The visible outer surface of the Sun is called the photosphere. Above this layer
is a thin region known as the chromosphere. This is surrounded by a transition
region of rapidly increasing temperatures, then by the super-heated corona.
At the center of the Sun is the core region, a volume of sufficient temperature
and pressure for nuclear fusion to occur. Above the core is the radiation zone,
where the plasma conveys the energy flux by means of radiation. The outer
layers form a convection zone where the gas material transports energy


primarily through physical displacement of the gas. It is believed that this
convection zone creates the magnetic activity that generates sun spots.
A solar wind of plasma particles constantly streams outward from the Sun until
it reaches the heliopause. This solar wind interacts with the magnetosphere of
the Earth to create the Van Allen radiation belts, as well as the aurora where the
lines of the Earth’s magnetic field descend into the atmosphere.11

Planetary Science
Planetary science is the scientific study of planets, moons, and planetary
systems, in particular those of the Solar System and the processes that form
them. It studies objects ranging in size from micrometeoroids to gas giants,
aiming to determine their composition, dynamics, formation, interrelations and
Planetary science examines the assemblage of planets, moons, dwarf planets,
comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar
planets. The solar system has been relatively well-studied, initially through
telescopes and then later by spacecraft. This has provided a good overall
understanding of the formation and evolution of this planetary system, although
many new discoveries are still being made.12
The solar system is subdivided into the inner planets, the asteroid belt, and the
outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and
Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.
Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may
extend as far as a light-year.13
The planets were formed by a protoplanetary disk that surrounded the early Sun.
Through a process that included gravitational attraction, collision, and accretion,
the disk formed clumps of matter that, with time, became protoplanets. The
radiation pressure of the solar wind then expelled most of the unaccreted matter,
and only those planets with sufficient mass retained their gaseous atmosphere.
The planets continued to sweep up, or eject, the remaining matter during a


period of intense bombardment, evidenced by the many impact craters on the
Moon. During this period, some of the protoplanets may have collided, the
leading hypothesis for how the Moon was formed.
Once a planet reaches sufficient mass, the materials with different densities
segregate within, during planetary differentiation. This process can form a stony
or metallic core, surrounded by a mantle and an outer surface. The core may
include solid and liquid regions, and some planetary cores generate their own
magnetic field, which can protect their atmospheres from solar wind stripping.
A planet or moon’s interior heat is produced from the collisions that created the
body, radioactive materials or tidal heating. Some planets and moons
accumulate enough heat to drive geologic processes such as volcanism and
tectonics. Those that accumulate or retain an atmosphere can also undergo
surface erosion from wind or water. Smaller bodies, without tidal heating, cool
more quickly; and their geological activity ceases with the exception of impact

Stellar Astronomy
The study of stars and stellar evolution is fundamental to our understanding of
the universe. The astrophysics of stars has been determined through observation
and theoretical understanding; and from computer simulations of the interior.
Star formation occurs in dense regions of dust and gas, known as giant
molecular clouds. When destabilized, cloud fragments can collapse under the
influence of gravity, to form a protostar. A sufficiently dense, and hot, core
region will trigger nuclear fusion, thus creating a main-sequence star. Almost all
elements heavier than hydrogen and helium were created inside the cores of
The characteristics of the resulting star depend primarily upon its starting mass.
The more massive the star, the greater its luminosity, and the more rapidly it
expends the hydrogen fuel in its core. Over time, this hydrogen fuel is
completely converted into helium, and the star begins to evolve. The fusion of


helium requires a higher core temperature, so that the star both expands in size,
and increases in core density. The resulting red giant enjoys a brief life span,
before the helium fuel is in turn consumed. Very massive stars can also undergo
a series of decreasing evolutionary phases, as they fuse increasingly heavier
The final fate of the star depends on its mass, with stars of mass greater than
about eight times the Sun becoming core collapse supernovae; while smaller
stars form planetary nebulae, and evolve into white dwarfs. The remnant of a
supernova is a dense neutron star, or, if the stellar mass was at least three times
that of the Sun, a black hole. Close binary stars can follow more complex
evolutionary paths, such as mass transfer onto a white dwarf companion that can
potentially cause a supernova. Planetary nebulae and supernovae are necessary
for the distribution of metals to the interstellar medium; without them, all new
stars (and their planetary systems) would be formed from hydrogen and helium

Galactic Astronomy
Our solar system orbits within the Milky Way, a barred spiral galaxy that is a
prominent member of the Local Group of galaxies. It is a rotating mass of gas,
dust, stars and other objects, held together by mutual gravitational attraction. As
the Earth is located within the dusty outer arms, there are large portions of the
Milky Way that are obscured from view.
In the center of the Milky Way is the core, a bar-shaped bulge with what is
believed to be a supermassive black hole at the center. This is surrounded by
four primary arms that spiral from the core. This is a region of active star
formation that contains many younger, population I stars. The disk is
surrounded by a spheroid halo of older, population II stars, as well as relatively
dense concentrations of stars known as globular clusters.
Between the stars lies the interstellar medium, a region of sparse matter. In the
densest regions, molecular clouds of molecular hydrogen and other elements


create star-forming regions. These begin as irregular dark nebulae, which
concentrate and collapse to form compact protostars.
As the more massive stars appear, they transform the cloud into an H II region
of glowing gas and plasma. The stellar wind and supernova explosions from
these stars eventually serve to disperse the cloud, often leaving behind one or
more young open clusters of stars. These clusters gradually disperse, and the
stars join the population of the Milky Way.
Kinematic studies of matter in the Milky Way and other galaxies have
demonstrated that there is more mass than can be accounted for by visible
matter. A dark matter halo appears to dominate the mass, although the nature of
this dark matter remains undetermined.17

Extragalactic Astronomy
The study of objects outside of our galaxy is a branch of astronomy concerned
with the formation and evolution of Galaxies; their morphology and
classification; and the examination of active galaxies, and the groups and
clusters of galaxies. The latter is important for the understanding of the largescale structure of the cosmos.
Most galaxies are organized into distinct shapes that allow for classification
schemes. They are commonly divided into spiral, elliptical and irregular
galaxies. As the name suggests, an elliptical galaxy has the cross-sectional
shape of an ellipse. The stars move along random orbits with no preferred
direction. These galaxies contain little or no interstellar dust; few star-forming
regions; and generally older stars. Elliptical galaxies are more commonly found
at the core of galactic clusters, and may be formed through mergers of large
A spiral galaxy is organized into a flat, rotating disk, usually with a prominent
bulge or bar at the center, and trailing bright arms that spiral outward. The arms
are dusty regions of star formation where massive young stars produce a blue


tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the
Milky Way and the Andromeda Galaxy are spiral galaxies.
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical.
About a quarter of all galaxies are irregular, and the peculiar shapes of such
galaxies may be the result of gravitational interaction.
An active galaxy is a formation that is emitting a significant amount of its
energy from a source other than stars, dust and gas; and is powered by a
compact region at the core, usually thought to be a super-massive black hole
that is emitting radiation from in-falling material.
A radio galaxy is an active galaxy that is very luminous in the radio portion of
the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies
that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars.
Quasars are believed to be the most consistently luminous objects in the known
The large-scale structure of the cosmos is represented by groups and clusters of
galaxies. This structure is organized in a hierarchy of groupings, with the largest
being the superclusters. The collective matter is formed into filaments and
walls, leaving large voids in between.19

Space Flight
Spaceflight is the use of space technology to achieve the flight of spacecraft into
and through outer space. The most commonly used definition of outer space is
everything beyond the Kármán line, which is 100 kilometers (62 mi) above the
Earth’s surface. The United States sometimes defines outer space as everything
beyond 50 miles (80 km) in altitude.20
Space missions are designed to explore the unknown and learn more about the
universe around us. Whether it’s a manned space mission to the moon, a robot
explorer in the solar system or a probe to the galaxy beyond.
Spaceflight is used in space exploration, and also in commercial activities like
space tourism and satellite telecommunications. Additional non-commercial


uses of spaceflight include space observatories, reconnaissance satellites and
other earth observation satellites.
A spaceflight typically begins with a rocket launch, which provides the initial
thrust to overcome the force of gravity and propels the spacecraft from the
surface of the Earth. Once in space, the motion of a spacecraft—both when
unpropelled and when under propulsion—is covered by the area of study called
astrodynamics. Some spacecraft remain in space indefinitely, some disintegrate
during atmospheric reentry, and others reach a planetary or lunar surface for
landing or impact.
The first rocket to reach space was the German V-2 Rocket, on a test flight in
June, 1944, although sub-orbital flight is not considered a spaceflight in Russia.
On October 4, 1957, the Soviet Union launched Sputnik 1, which became the
first artificial satellite to orbit the Earth. The first human spaceflight was Vostok
1 on April 12, 1961, aboard which Soviet cosmonaut Yuri Gagarin made one
orbit around the Earth. The lead architects behind the Soviet space program’s
Vostok 1 mission were the rocket scientists Sergey Korolyov and Kerim
Rockets remain the only currently practical means of reaching space. Other nonrocket spacelaunch technologies such as scramjets still fall far short of orbital
Spacecraft are vehicles capable of controlling their trajectory through space.
The first ‘true spacecraft’ is sometimes said to be Apollo Lunar Module, since
this was the only manned vehicle to have been designed for, and operated only
in space; and is notable for its non aerodynamic shape.22

Human Space Flight
The first human spaceflight was Vostok 1 on April 12, 1961, on which
cosmonaut Yuri Gagarin of the USSR made one orbit around the Earth. In
official Soviet documents, there is no mention of the fact that Gagarin
parachuted the final seven miles. The international rules for aviation records


stated that “The pilot remains in his craft from launch to landing”. This rule, if
applied, would have disqualified Gagarin’s space-flight. Currently the only
spacecraft regularly used for human spaceflight are Russian Soyuz spacecraft
and the U.S. Space Shuttle fleet. Each of those space programs has used other
spacecraft in the past. Recently, the Shenzhou spacecraft has been used twice
for human spaceflight, as has SpaceshipOne.
In a microgravity environment such as that provided by a spacecraft in orbit
around the Earth, humans experience a sense of weightlessness. Short-term
exposure to microgravity causes space adaptation syndrome, a self-limiting
nausea caused by derangement of the vestibular system. Long-term exposure
causes multiple health issues. The most significant is bone loss, some of which
is permanent, but microgravity also leads to significant deconditioning of
muscular and cardiovascular tissues. Once above the atmosphere, radiation due
to the Van Allen belts, solar radiation and cosmic radiation issues occur and
increase. Further away from the Earth, solar flares can give a fatal radiation
dose in minutes, and cosmic radiation would significantly increase the chances
of cancer over a decade exposure or more.23, 24
In human spaceflight, the life support system is a group of devices that allow a
human being to survive in outer space. NASA often uses the phrase
Environmental Control and Life Support System or the acronym ECLSS when
describing these systems for its human spaceflight missions. The life support
system may supply: air, water and food. It must also maintain the correct body
temperature, an acceptable pressure on the body and deal with the body’s waste
products. Shielding against harmful external influences such as radiation and
micro-meteorites may also be necessary. Components of the life support system
are life-critical, and are designed and constructed using safety engineering
Five spacecraft are currently leaving the Solar System on escape trajectories.
The one farthest from the Sun is Voyager 1, which is more than 100 AU distant


and is moving at 3.6 AU per year. In comparison Proxima Centauri, the closest
star other than the Sun, is 267,000 AU distant. It will take Voyager 1 over
74,000 years to reach this distance. Vehicle designs using other techniques, such
as nuclear pulse propulsion are likely to be able to reach the nearest star
significantly faster.
Another possibility that could allow for human interstellar spaceflight is to
make use of time dilation, as this would make it possible for passengers in a
fast-moving vehicle to travel further into the future while aging very little, in
that their great speed slows down the rate of passage of on-board time.
However, attaining such high speeds would still require the use of some new,
advanced method of propulsion.24
Intergalactic travel involves spaceflight between galaxies, and is considered
much more technologically demanding than even interstellar travel and, by
current engineering terms, is considered science fiction.
Current spaceflights are frequently, but not invariably paid for by governments.
But there are strong launch markets such as satellite television that is purely
commercial, although the launchers themselves are often at least partly funded
by governments.
Uses for spaceflight include earth observation satellites, space exploration,
space tourism, communication satellites and satellite navigation.
There is growing interest in spacecraft and flights paid for by commercial
companies and even private individuals. It is thought that some of the high cost
of access to space is due to governmental inefficiencies; and certainly the costs
of the governmental paperwork surrounding NASA is legendary. If a
commercial company were able to be more efficient, costs could come down
significantly. Space launch vehicles such as Falcon I have been wholly
developed with private finance and the quoted costs for launch are lower.24


Theoretical astronomers use a wide variety of tools, which include analytical
models (for example, polytropes to approximate the behaviors of a star) and
computational numerical simulations. Each has some advantages. Analytical
models of a process are generally better for giving insight into the heart of what
is going on. Numerical models can reveal the existence of phenomena and
effects that would otherwise not be seen.
Theorists in astronomy endeavor to create theoretical models and figure out the
observational consequences of those models. This helps observer’s look for data
that can refute a model or help in choosing between several alternate or
conflicting models.
Theorists also try to generate or modify models to take into account new data. In
the case of an inconsistency, the general tendency is to try to make minimal
modifications to the model to fit the data. In some cases, a large amount of
inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astronomers include: stellar dynamics and
evolution; galaxy formation; large-scale structure of matter in the Universe;
origin of cosmic rays; general relativity and physical cosmology, including
string cosmology and astroparticle physics. Astrophysical relativity serves as a
tool to gauge the properties of large scale structures for which gravitation plays
a significant role in physical phenomena investigated and as the basis for black
hole (astro)physics and the study of gravitational waves.25
Some widely accepted and studied theories and models in astronomy, now
included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark
matter, and fundamental theories of physics. Dark matter and dark energy are
the current leading topics in astronomy, as their discovery and controversy
originated during the study of the galaxies.


Observational astronomy is a division of the astronomical science that is
concerned with getting data, in contrast with theoretical astrophysics which is
mainly concerned with finding out the measurable implications of physical
models. It is the practice of observing celestial objects by using telescopes and
other astronomical apparatus.25
As a science, astronomy is somewhat hindered in that direct experiments with
the properties of the distant universe are not possible. However, this is partly
compensated by the fact that astronomers have a vast number of visible
examples of stellar phenomena that can be examined. This allows for
observational data to be plotted on graphs, and general trends recorded. Nearby
examples of specific phenomena, such as variable stars, can then be used to
infer the behavior of more distant representatives. Those distant yardsticks can
then be employed to measure other phenomena in that neighborhood, including
the distance to a galaxy.
A telescope is an instrument designed for the observation of remote objects by
the collection of electromagnetic radiation. The first known practically
functioning telescopes were invented in the Netherlands at the beginning of the
17th century. Telescopes can refer to a whole range of instruments operating in
most regions of the electromagnetic spectrum.26
The word telescope from the Greek tele = far and skopein = to look or see;
teleskopos = far-seeing, was coined in 1611 by the Greek mathematician
Giovanni Demisiani for one of Galileo Galilei’s instruments presented at a
banquet at the Accademia dei Lincei. In the Starry Messenger Galileo had used
the term Perspicillum. The earliest evidence of working telescopes was the
refracting telescopes that appeared in the Netherlands in 1608. Their
development is credited to three individuals: Hans Lippershey and Zacharias
Janssen, who were spectacle makers in Middelburg, and Jacob Metius of
Alkmaar. Galileo greatly improved upon these designs the following year.


Optical science is relevant to and studied in many related disciplines including
astronomy, various engineering fields, photography, ophthalmology and
optometry. Practical applications of optics are found in a variety of technologies
and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers,
and fiber optics.
Optics is the study of the behavior and properties of light, including its
interactions with matter and the construction of instruments that use or detect it.
Optics usually describes the behavior of visible, ultraviolet, and infrared light.
Because light is an electromagnetic wave, other forms of electromagnetic
radiation such as X-rays, microwaves, and radio waves exhibit similar
properties. 27
Most optical phenomena can be accounted for using the classical
electromagnetic description of light. Complete electromagnetic descriptions of
light are, however, often difficult to apply in practice. Practical optics is usually
done using simplified models. The most common of these, geometric optics,
treats light as a collection of rays that travel in straight lines and bend when they
pass through or reflect from surfaces. Physical optics is a more comprehensive
model of light, which includes wave effects such as diffraction and interference
that cannot be accounted for in geometric optics. Historically, the ray-based
model of light was developed first, followed by the wave model of light.
Progress in electromagnetic theory in the 19th century led to the discovery that
light waves were in fact electromagnetic radiation.28, 29
Optics began with the development of lenses by the ancient Egyptians and
Mesopotamians. The earliest known lenses were made from polished crystal,
and have been dated as early as 700 BC for Assyrian lenses such as the Nimrud
lens. The ancient Romans and Greeks filled glass spheres with water to make
lenses. These practical developments were followed by the development of
theories of light and vision by ancient Greek and Indian philosophers, and the


development of geometrical optics in the Greco-Roman world. The word optics
comes from the ancient Greek word meaning appearance or look. Plato first
articulated his emission theory, the idea that visual perception is accomplished
by rays of light emitted by the eyes and commented on the parity reversal of
mirrors in Timaeus. Some hundred years later, Euclid wrote a treatise entitled
Optics wherein he describes the mathematical rules of perspective and describes
the effects of refraction qualitatively. Ptolemy, in his treatise Optics,
summarizes much of Euclid and goes on to describe a way to measure the angle
of refraction, though he failed to notice the empirical relationship between it and
the angle of incidence.28
Al-Kindi (c. 801–73) was one of the earliest important writers on optics in the
Islamic world. In a work known in the West as De radiis stellarum, al-Kindi
resurrected Plato’s emission theory, which had an influence on later Western
scholars such as Robert Grosseteste and Roger Bacon. In 984, the Persian
mathematician, Ibn Sahl wrote a treatise “On Burning Mirrors and Lenses”,
correctly describing a law of refraction mathematically equivalent to Snell’s
law. He used his law of refraction to compute the shapes of lenses and mirrors
that focus light at a single point on the axis. In the early 11th century, Alhazen
(Ibn al-Haytham) wrote his Book of Optics, which extensively documented the
then-current Islamic understanding of optics and revolutionized the field. It
included the first descriptions of optical phenomena associated with pinholes
and concave lenses, provided the first correct explanation of vision, described
various experiments using an early scientific method, and greatly influenced the
later development of the modern telescope.29, 30
In the 13th century, Roger Bacon, inspired by Ibn al-Haytham, used parts of
glass spheres as magnifying glasses, and discovered that light reflects from
objects rather than being released from them. In Italy, around 1284, Salvino
D’Armate invented the first wearable eyeglasses. The first rudimentary


telescopes were developed independently in the 1570s and 1580s by Leonard
Digges, Taqi al-Din and Giambattista Della Porta.
The earliest known working telescopes were refracting telescopes, a type which
relies entirely on lenses for magnification. Their development in the
Netherlands in 1608 was by three individuals: Hans Lippershey and Zacharias
Janssen, who were spectacle makers in Middelburg, Holland, and Jacob Metius
of Alkmaar. In Italy, Galileo greatly improved upon these designs the following
year. In 1668, Isaac Newton constructed the first practical reflecting telescope,
which bears his name, the Newtonian reflector.
Optical theory progressed in the mid-17th century with treatises written by
philosopher René Descartes, which explained a variety of optical phenomena
including reflection and refraction by assuming that light was emitted by objects
which produced it. This differed substantively from ancient Greek notions that
light emanated from the eye. In the late 1660s and early 1670s, Newton
expanded Descartes’ ideas into a corpuscle theory of light, famously showing
that white light, instead of being a unique color, was really a composite of
different colors that can be separated into a spectrum with a prism. In 1690,
Christian Huygens proposed a wave theory for light based on suggestions that
had been made by Robert Hooke in 1664. Hooke himself publicly criticized
Newton’s theories of light and the feud between the two lasted until Hooke’s
death. In 1704, Newton published Opticks and, at the time, partly because of his
success in other areas of physics, he was generally considered to be the victor in
the debate over the nature of light.30-32
Newtonian optics and emission theory was generally accepted until the early
19th century when Thomas Young and Augustin-Jean Fresnel conducted
experiments on the interference of light that firmly established light’s wavenature. Young’s famous double slit experiment showed that light followed the
law of superposition, something normal particles do not follow. This work led
to a theory of diffraction for light and opened an entire area of study in physical


optics. Wave optics was successfully unified with electromagnetic theory by
James Clerk Maxwell in the 1860s.32-33
The next development in optical theory came in 1899 when Max Planck
correctly modeled blackbody radiation by assuming that the exchange of energy
between light and matter only occurred in discrete amounts he called quanta. In
1905, Albert Einstein published the theory of the photoelectric effect that firmly
established the quantization of light itself. In 1913, Niels Bohr showed that
atoms could only emit discrete amounts of energy, thus explaining the discrete
lines seen in emission and absorption spectra. The understanding of the
interaction between light and matter, which followed from these developments,
not only formed the basis of quantum optics but also was crucial for the
development of quantum mechanics as a whole. The ultimate culmination was
the theory of quantum electrodynamics, which explains all optics and
electromagnetic processes in general as being the result of the exchange of real
and virtual photons.33, 34
Quantum optics gained practical importance with the invention of the maser in
1953 and the laser in 1960. Following the work of Paul Dirac in quantum field
theory, George Sudarshan, Roy J. Glauber, and Leonard Mandel applied
quantum theory to the electromagnetic field in the 1950s and 1960s to gain a
more detailed understanding of photodetection and the statistics of light.35

Optical Telescopes
The name ‘telescope’ covers a wide range of instruments and is difficult to
define. They all have the attribute of collecting electromagnetic radiation. The
most common type is the optical telescope.
An optical telescope gathers and focuses light mainly from the visible part of
the electromagnetic. Optical telescopes increase the apparent angular size of
distant objects as well as their apparent brightness. In order for the image to be
observed, photographed, studied, and sent to a computer, telescopes work by
employing one or more curved optical elements—usually made from glass—


lenses, or mirrors to gather light and other electromagnetic radiation to bring
that light or radiation to a focal point. Optical telescopes are used for astronomy
and in many non-astronomical instruments including spotting scopes,
binoculars, camera lenses, and spyglasses. Optical Telescopes come in three
basic designs; Refractor, Reflector, and Catadioptric.36
The refracting telescope which uses lenses to form an image. The reflecting
telescope which uses an arrangement of mirrors to form an image. The
Catadioptric telescope which uses mirrors combined with lenses to form an
image. A refractor uses two lenses. At one end (the end farther away from the
viewer), is the larger lens, called the objective lens or object glass. On the other
end is the lens for look through. It is called the ocular or eyepiece.
The objective collects light and focuses it as a sharp image. This image is
magnified and seen through the ocular. The eyepiece is adjusted by sliding it in
and out of the telescope body to focus the image.
A reflector works a bit differently. Light is gathered at the bottom of the scope
by a concave mirror, called the Primary. The primary has a parabolic shape.
There are several ways the primary can focus the light, and how it is done
determines the type of reflecting telescope.
Many observatory Telescopes use a photographic plate to focus the image.
Called the Prime Focus Position, the plate is located near the top of the scope.
Other scopes use a secondary mirror, placed in a similar position as the
photographic plate, to reflect the image back down the body of the scope, where
it is viewed through a hole in the primary mirror. This is known as a Cassegrain
The Newtonian telescope, a kind of reflector. So named because Sir Isaac
Newton created the basic design. In a Newtonian, a flat mirror is placed at an
angle in the same position as the secondary mirror in a Cassegrain. This
secondary mirror focuses the image into an eyepiece located in the side of the
tube, near the top of the scope.37


The Catadioptric telescopes combine elements of refractors and reflectors in
their design. The first such telescope was created by German astronomer
Bernhard Schmidt in 1930. It used a primary mirror at the back of the telescope
with a glass corrector plate in the front of the telescope, which was designed to
remove spherical aberration. In the original telescope, photographic film was
placed at the prime focus. There were no secondary mirrors or eyepieces. The
descendant of that original design, called the Schmidt-Cassegrain design, is the
most popular type of telescope. Invented in the 1960s, it has a secondary mirror
that bounces light through a hole in the primary mirror to an eyepiece.

Radio Telescopes
Radio telescopes are directional radio antennas that often have a parabolic
shape. The dishes are sometimes constructed of a conductive wire mesh whose
openings are smaller than the wavelength being observed. Multi-element Radio
telescopes are constructed from pairs or larger groups of these dishes to
synthesize large ‘virtual’ apertures that are similar in size to the separation
between the telescopes; this process is known as aperture synthesis. As of 2005,
the current record array size is many times the width of the Earth—utilizing
space-based Very Long Baseline Interferometry (VLBI) telescopes such as the
Japanese HALCA (Highly Advanced Laboratory for Communications and
Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture
synthesis is now also being applied to optical telescopes using optical








interferometry at single reflecting telescopes. Radio telescopes are also used to
collect microwave radiation, which is used to collect radiation when any visible
light is obstructed or faint, such as from quasars. Some radio telescopes are used
by programs such as SETI and the Arecibo Observatory to search for
exterrestrial life. One particularly exciting example is the Wow! Signal,
recorded in 1977.38, 39


High Energy Particle Telescopes
High-energy astronomy requires specialized telescopes to make observations
since most of these particles go through most metals and glasses. X-ray
telescopes use Wolter telescopes composed of ring-shaped ‘glancing’ mirrors
made of heavy metals that are able to reflect the rays just a few degrees. The
mirrors are usually a section of a rotated parabola and a hyperbola, or ellipse. In
1952, Hans Wolter outlined 3 ways a telescope could be built using only this
kind of mirror. Gamma-ray telescopes refrain from focusing completely and use
coded aperture masks: the patterns of the shadow the mask creates can be
reconstructed to form an image.40
X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or
high-flying balloons since the Earth’s atmosphere is opaque to this part of the
electromagnetic spectrum. In other types of high energy particle telescopes there
is no image-forming optical system. Cosmic-ray telescopes usually consist of an
array of different detector types spread out over a large area. A Neutrino
telescope consists of a large mass of water or ice, surrounded by an array of
sensitive light detectors known as photomultiplier tubes.41

The Solar System consists of the Sun and those celestial objects bound to it by
gravity, all of which formed from the collapse of a giant molecular cloud
approximately 4.6 billion years ago. The Sun’s retinue of objects circle it in a
nearly flat disc called the ecliptic plane, most of the mass of which is contained
within eight relatively solitary planets whose orbits are almost circular. The four
smaller inner planets; Mercury, Venus, Earth and Mars, also called the
terrestrial planets, are primarily composed of rock and metal. The four outer
planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are
composed largely of hydrogen and helium and are far more massive than the
terrestrials.42, 43


The Solar System is also home to two main belts of small bodies. The asteroid
belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as
it is composed mainly of rock and metal. The Kuiper belt (and its
subpopulation, the scattered disc), which lies beyond Neptune’s orbit, is
composed mostly of ices such as water, ammonia and methane. Within these
belts, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are
recognised to be large enough to have been rounded by their own gravity, and
are thus termed dwarf planets. The hypothetical Oort cloud, which acts as the
source for long-period comets, may also exist at a distance roughly a thousand
times beyond these regions.43
Within the Solar System, various populations of small bodies, such as comets,
centaurs and interplanetary dust, freely travel between these regions, while the
solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar
medium known as the heliosphere, which extends out to the edge of the
scattered disc.
Six of the planets and three of the dwarf planets are orbited by natural satellites,
usually termed moons after Earth’s Moon. Each of the outer planets is encircled
by planetary rings of dust and other particles.

Discovery and Exploration
For many thousands of years, humanity, with a few notable exceptions, did not
recognise the existence of the Solar System. They believed the Earth to be
stationary at the centre of the universe and categorically different from the
divine or ethereal objects that moved through the sky. Although the Indian
mathematician-astronomer Aryabhata and the Greek philosopher Aristarchus of
Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus
Copernicus was the first to develop a mathematically predictive heliocentric
system. His 17th-century successors Galileo Galilei, Johannes Kepler, and Isaac
Newton developed an understanding of physics which led to the gradual
acceptance of the idea that the Earth moves around the Sun and that the planets


are governed by the same physical laws that governed the Earth. In more recent
times, improvements in the telescope and the use of unmanned spacecraft have
enabled the investigation of geological phenomena such as mountains and
craters and seasonal meteorological phenomena such as clouds, dust storms and
ice caps on the other planets.43, 44

The principal component of the Solar System is the Sun, a main sequence G2
star that contains 99.86 percent of the system’s known mass and dominates it
gravitationally. The Sun’s four largest orbiting bodies, the gas giants, account
for 99 percent of the remaining mass, with Jupiter and Saturn together
comprising more than 90 percent. Most large objects in orbit around the Sun lie
near the plane of Earth’s orbit, known as the ecliptic. The planets are very close
to the ecliptic while comets and Kuiper belt objects are frequently at
significantly greater angles to it.43, 45
All of the planets and most other objects also orbit with the Sun’s rotation
(counter-clockwise, as viewed from above the Sun’s North Pole). There are
exceptions, such as Halley’s Comet.
To cope with the vast distances involved, many representations of the Solar
System show orbits the same distance apart. In reality, with a few exceptions,
the farther a planet or belt is from the Sun, the larger the distance between it and
the previous orbit. For example, Venus is approximately 0.33 astronomical units
(AU) farther out than Mercury, while Saturn is 4.3 AU out from Jupiter, and
Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine
a correlation between these orbital distances (see Titius-Bode law), but no such
theory has been accepted.44
Kepler’s laws of planetary motion describe the orbits of objects about the Sun.
According to Kepler’s laws, each object travels along an ellipse with the Sun at
one focus. Objects closer to the Sun (with smaller semi-major axes) have shorter
years. On an elliptical orbit, a body’s distance from the Sun varies over the


course of its year. A body’s closest approach to the Sun is called its perihelion,
while its most distant point from the Sun is called its aphelion. Each body
moves fastest at its perihelion and slowest at its aphelion. The orbits of the
planets are nearly circular, but many comets, asteroids and Kuiper belt objects
follow highly elliptical orbits.
Most of the planets in the Solar System possess secondary systems of their own.
Many are in turn orbited by planetary objects called natural satellites, or moons,
some of which are larger than planets. Most of the largest natural satellites are
in synchronous rotation, with one face permanently turned toward their parent.
The four largest planets, the gas giants, also possess planetary rings, thin bands
of tiny particles that orbit them in unison.46

The Sun
The Sun is the Solar System’s star, and far and away its chief component. Its
large mass (332,900 Earth masses) produces temperatures and densities in its
core great enough to sustain nuclear fusion, which releases enormous amounts
of energy, mostly radiated into space as electromagnetic radiation, peaking in
the 400–to–700 nm band we call visible light.
The Sun is classified as a type G2 yellow dwarf, but this name is misleading as,
compared to majority of stars in our galaxy, the Sun is rather large and bright.
Stars are classified by the Hertzsprung-Russell diagram, a graph which plots the
brightness of stars against their surface temperatures. Generally, hotter stars are
brighter. Stars following this pattern are said to be on the main sequence, and
the Sun lies right in the middle of it. However, stars brighter and hotter than the
Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs,
are common, making up 85 percent of the stars in the galaxy.47
It is believed that the Sun’s position on the main sequence puts it in the prime of
life for a star, in that it has not yet exhausted its store of hydrogen for nuclear
fusion. The Sun is growing brighter; early in its history it was 70 percent as
bright as it is today.


The Sun is a population I star; it was born in the later stages of the universe’s
evolution, and thus contains more elements heavier than hydrogen and helium
(metals in astronomical parlance) than older population II stars. Elements
heavier than hydrogen and helium were formed in the cores of ancient and
exploding stars, so the first generation of stars had to die before the universe
could be enriched with these atoms. The oldest stars contain few metals, while
stars born later have more. This high metallicity is thought to have been crucial
to the Sun’s developing a planetary system, because planets form from accretion
of metals.43, 48

Interplanetary Medium
Along with light, the Sun radiates a continuous stream of charged particles (a
plasma) known as the solar wind. This stream of particles spreads outwards at
roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the
heliosphere) that permeates the Solar System out to at least 100 AU. This is
known as the interplanetary medium. Geomagnetic storms on the Sun’s surface,
such as solar flares and coronal mass ejections, disturb the heliosphere, creating
space weather. The largest structure within the heliosphere is the heliospheric
current sheet, a spiral form created by the actions of the Sun’s rotating magnetic
field on the interplanetary medium.49
Earth’s magnetic field stops its atmosphere from being stripped away by the
solar wind. Venus and Mars do not have magnetic fields, and as a result, the
solar wind causes their atmospheres to gradually bleed away into space. Coronal
mass ejections and similar events, blow magnetic field and huge quantities of
material from the surface of the Sun. The interaction of this magnetic field and
material with Earth’s magnetic field funnels charged particles into the Earth’s
upper atmosphere, where its interactions create aurorae seen near the magnetic
Cosmic rays originate outside the Solar System. The heliosphere partially
shields the Solar System, and planetary magnetic fields (for those planets that


have them) also provide some protection. The density of cosmic rays in the
interstellar medium and the strength of the Sun’s magnetic field change on very
long timescales, so the level of cosmic radiation in the Solar System varies,
though by how much is unknown. The interplanetary medium is home to at least
two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in
the inner Solar System and causes zodiacal light. It was likely formed by
collisions within the asteroid belt brought on by interactions with the planets.
The second extends from about 10 AU to about 40 AU, and was probably
created by similar collisions within the Kuiper belt.51, 52

Inner Solar System
The inner Solar System is the traditional name for the region comprising the
terrestrial planets and asteroids. Composed mainly of silicates and metals, the
objects of the inner Solar System are relatively close to the Sun; the radius of
this entire region is shorter than the distance between Jupiter and Saturn.
The four inner or terrestrial planets have dense, rocky compositions, few or no
moons, and no ring systems. They are composed largely of refractory minerals,
such as the silicates which form their crusts and mantles, and metals such as
iron and nickel, which form their cores. Three of the four inner planets (Venus,
Earth and Mars have substantial atmospheres; all have impact craters and
tectonic surface features such as rift valleys and volcanoes. The term inner
planet should not be confused with inferior planet, which designates those
planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).53, 54
Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055
Earth masses). Mercury has no natural satellites, and it’s only known geological
features besides impact craters are lobed ridges or rupes, probably produced by
a period of contraction early in its history. Mercury’s almost negligible
atmosphere consists of atoms blasted off its surface by the solar wind. Its
relatively large iron core and thin mantle have not yet been adequately
explained. Hypotheses include that its outer layers were stripped off by a giant


impact, and that it was prevented from fully accreting by the young Sun’s
Venus (0.7 AU) is close in size to Earth, (0.815 Earth masses) and like Earth,
has a thick silicate mantle around an iron core, a substantial atmosphere and
evidence of internal geological activity. However, it is much drier than Earth
and its atmosphere is ninety times as dense. Venus has no natural satellites. It is
the hottest planet, with surface temperatures over 400°C, most likely due to the
amount of greenhouse gases in the atmosphere. No definitive evidence of
current geological activity has been detected on Venus, but it has no magnetic
field that would prevent depletion of its substantial atmosphere, which suggests
that its atmosphere is regularly replenished by volcanic eruptions.54
Earth (1 AU) is the largest and densest of the inner planets, the only one known
to have current geological activity, and is the only place in the universe where
life is known to exist. Its liquid hydrosphere is unique among the terrestrial
planets, and it is also the only planet where plate tectonics has been observed.
Earth’s atmosphere is radically different from those of the other planets, having
been altered by the presence of life to contain 21% free oxygen. It has one
natural satellite, the Moon, the only large satellite of a terrestrial planet in the
Solar System.55
Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It
possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1
millibars (roughly 0.6 percent that of the Earth’s). Its surface, peppered with
vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris,
shows geological activity that may have persisted until as recently as 2 million
years ago. Its red colour comes from iron oxide (rust) in its soil. Mars has two
tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.56
Asteroids are mostly small Solar System bodies composed mainly of refractory
rocky and metallic minerals. The main asteroid belt occupies the orbit between
Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be


remnants from the Solar System’s formation that failed to coalesce because of
the gravitational interference of Jupiter.
Asteroids range in size from hundreds of kilometres across to microscopic. All
asteroids save the largest, Ceres, are classified as small Solar System bodies, but
some asteroids such as Vesta and Hygieia may be reclassed as dwarf planets if
they are shown to have achieved hydrostatic equilibrium.
The asteroid belt contains tens of thousands, possibly millions, of objects over
one kilometre in diameter. Despite this, the total mass of the main belt is
unlikely to be more than a thousandth of that of the Earth. The main belt is very
sparsely populated; spacecraft routinely pass through without incident.
Asteroids with diameters between 10 and 10−4 m are called meteoroids.57
Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a
dwarf planet. It has a diameter of slightly under 1000 km, and a mass large
enough for its own gravity to pull it into a spherical shape. Ceres was
considered a planet when it was discovered in the 19th century, but was
reclassified as an asteroid in the 1850s as further observation revealed additional
asteroids. It was again reclassified in 2006 as a dwarf planet.57
Asteroids in the main belt are divided into asteroid groups and families based on
their orbital characteristics. Asteroid moons are asteroids that orbit larger
asteroids. They are not as clearly distinguished as planetary moons, sometimes
being almost as large as their partners. The asteroid belt also contains main-belt
comets which may have been the source of Earth’s water.
Trojan asteroids are located in either of Jupiter’s L4 or L5 points
(gravitationally stable regions leading and trailing a planet in its orbit); the term
Trojan is also used for small bodies in any other planetary or satellite Lagrange
point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around
the Sun three times for every two Jupiter orbits. The inner Solar System is also
dusted with rogue asteroids, many of which cross the orbits of the inner
planets.57, 58


Outer Solar System
The outer region of the Solar System is home to the gas giants and their large
moons. Many short period comets, including the centaurs, also orbit in this
region. Due to their greater distance from the Sun, the solid objects in the outer
Solar System are composed of a higher proportion of ices (such as water,
ammonia, methane, often called ices in planetary science) than the rocky
denizens of the inner Solar System, as the colder temperatures allow these
compounds to remain solid.
The four outer planets, or gas giants (sometimes called Jovian planets),
collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and
Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune
possess a greater proportion of ices in their makeup. Some astronomers suggest
they belong in their own category, ice giants. All four gas giants have rings,
although only Saturn’s ring system is easily observed from Earth. The term
outer planet should not be confused with superior planet, which designates
planets outside Earth’s orbit.58, 59
Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times all the mass of all the other
planets put together. It is composed largely of hydrogen and helium. Jupiter’s
strong internal heat creates a number of semi-permanent features in its
atmosphere, such as cloud bands and the Great Red Spot. Jupiter has sixty-three
known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show
similarities to the terrestrial planets, such as volcanism and internal heating.
Ganymede, the largest satellite in the Solar System, is larger than Mercury.58
Saturn (9.5 AU), distinguished by its extensive ring system, has several
similarities to Jupiter, such as its atmospheric composition and magnetosphere.
Although Saturn has 60% of Jupiter’s volume, it is less than a third as massive,
at 95 Earth masses, making it the least dense planet in the Solar System. Saturn
has sixty confirmed satellites; two of which, Titan and Enceladus, show signs of
geological activity, though they are largely made of ice. Titan is larger than


Mercury and the only satellite in the Solar System with a substantial
Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets.
Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over
ninety degrees to the ecliptic. It has a much colder core than the other gas
giants, and radiates very little heat into space. Uranus has twenty-seven known
satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.58
Neptune (30 AU), though slightly smaller than Uranus, is more massive
(equivalent to 17 Earths) and therefore more dense. It radiates more internal
heat, but not as much as Jupiter or Saturn. Neptune has thirteen known
satellites. The largest, Triton, is geologically active, with geysers of liquid
nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is
accompanied in its orbit by a number of minor planets, termed Neptune Trojans
that are in 1:1 resonance with it.59
Comets are small Solar System bodies, typically only a few kilometres across,
composed largely of volatile ices. They have highly eccentric orbits, generally a
perihelion within the orbits of the inner planets and an aphelion far beyond
Pluto. When a comet enters the inner Solar System, its proximity to the Sun
causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas
and dust often visible to the naked eye.60
Short-period comets have orbits lasting less than two hundred years. Longperiod comets have orbits lasting thousands of years. Short-period comets are
believed to originate in the Kuiper belt, while long-period comets, such as HaleBopp, are believed to originate in the Oort cloud. Many comet groups, such as
the Kreutz Sungrazers, formed from the breakup of a single parent. Some
comets with hyperbolic orbits may originate outside the Solar System, but
determining their precise orbits is difficult. Old comets that have had most of
their volatiles driven out by solar warming are often categorised as asteroids.61


Farthest Regions
The point at which the Solar System ends and interstellar space begins is not
precisely defined, since its outer boundaries are shaped by two separate forces:
the solar wind and the Sun’s gravity. The outer limit of the solar wind’s
influence is roughly four times Pluto’s distance from the Sun; this heliopause is
considered the beginning of the interstellar medium. However, the Sun’s Roche
sphere, the effective range of its gravitational influence, is believed to extend up
to a thousand times farther.62
The heliosphere is divided into two separate regions. The solar wind travels at
roughly 400 km/s until it collides with the interstellar wind; the flow of plasma
in the interstellar medium. The collision occurs at the termination shock, which
is roughly 80–100 AU from the Sun upwind of the interstellar medium and
roughly 200 AU from the Sun downwind. Here the wind slows dramatically,
condenses and becomes more turbulent, forming a great oval structure known as
the heliosheath that looks and behaves very much like a comet’s tail, extending
outward for a further 40 AU on the upwind side but tailing many times that
distance downwind. Both Voyager 1 and Voyager 2 are reported to have passed
the termination shock and entered the heliosheath, at 94 and 84 AU from the
Sun, respectively. The outer boundary of the heliosphere, the heliopause, is the
point at which the solar wind finally terminates and is the beginning of
interstellar space.
The shape and form of the outer edge of the heliosphere is likely affected by the
fluid dynamics of interactions with the interstellar medium as well as solar
magnetic fields prevailing to the south, e.g. it is bluntly shaped with the
northern hemisphere extending 9 AU (roughly 900 million miles) farther than
the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the
bow shock, a plasma wake left by the Sun as it travels through the Milky Way.63
No spacecraft have yet passed beyond the heliopause, so it is impossible to
know for certain the conditions in local interstellar space. It is expected that


NASA’s Voyager spacecraft will pass the heliopause some time in the next
decade and transmit valuable data on radiation levels and solar wind back to the
Earth. How well the heliosphere shields the Solar System from cosmic rays is
poorly understood. A NASA-funded team has developed a concept of a Vision
Mission dedicated to sending a probe to the heliosphere.64
Much of our Solar System is still unknown. The Sun’s gravitational field is
estimated to dominate the gravitational forces of surrounding stars out to about
two light years (125,000 AU). Lower estimates for the radius of the Oort cloud,
by contrast, do not place it farther than 50,000 AU. Despite discoveries such as
Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of
thousands of AU in radius, is still virtually unmapped. There are also ongoing
studies of the region between Mercury and the Sun. Objects may yet be
discovered in the Solar System’s uncharted regions.62
The Solar System is located in the Milky Way galaxy, a barred spiral galaxy
with a diameter of about 100,000 light-years containing about 200 billion stars.
Our Sun resides in one of the Milky Way’s outer spiral arms, known as the
Orion Arm or Local Spur. The Sun lies between 25,000 and 28,000 light years
from the Galactic Centre, and its speed within the galaxy is about 220
kilometres per second, so that it completes one revolution every 225–250
million years. This revolution is known as the Solar System’s cosmic year. The
solar apex, the direction of the Sun’s path through interstellar space, is near the
constellation of Hercules in the direction of the current location of the bright
star Vega.65-67
The Solar System’s location in the galaxy is very likely a factor in the evolution
of life on Earth. Its orbit is close to being circular and is at roughly the same
speed as that of the spiral arms, which means it passes through them only rarely.
Since spiral arms are home to a far larger concentration of potentially dangerous
supernovae, this has given Earth long periods of interstellar stability for life to
evolve. The Solar System also lies well outside the star-crowded environs of the


galactic centre. Near the centre, gravitational tugs from nearby stars could
perturb bodies in the Oort Cloud and send many comets into the inner Solar
System, producing collisions with potentially catastrophic implications for life
on Earth. The intense radiation of the galactic centre could also interfere with
the development of complex life. Even at the Solar System’s current location,
some scientists have hypothesised that recent supernovae may have adversely
affected life in the last 35,000 years by flinging pieces of expelled stellar core
towards the Sun in the form of radioactive dust grains and larger, comet-like
bodies.68, 69
The immediate galactic neighbourhood of the Solar System is known as the
Local Interstellar Cloud or Local Fluff, an area of dense cloud in an otherwise
sparse region known as the Local Bubble, an hourglass-shaped cavity in the
interstellar medium roughly 300 light years across. The bubble is suffused with
high-temperature plasma that suggests it is the product of several recent
There are relatively few stars within ten light years (95 trillion km) of the Sun.
The closest is the triple star system Alpha Centauri, which is about 4.4 light
years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars,
while the small red dwarf Alpha Centauri C (also known as Proxima Centauri)
orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun
are the red dwarfs Barnard’s Star (at 5.9 light years), Wolf 359 (7.8 light years)
and Lalande 21185 (8.3 light years). The largest star within ten light years is
Sirius, a bright main sequence star roughly twice the Sun’s mass and orbited by
a white dwarf called Sirius B. It lies 8.6 light years away. The remaining
systems within ten light years are the binary red dwarf system Luyten 726-8
(8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years). Our
closest solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has
roughly 80 percent the Sun’s mass, but only 60 percent its luminosity.71, 72


The closest known extrasolar planet to the Sun lies around the star Epsilon
Eridani, a star slightly dimmer and redder than the Sun, which lies 10.5 light
years away. Its one confirmed planet, Epsilon Eridani b, is roughly 1.5 times
Jupiter’s mass and orbits its star every 6.9 years.

Formation and Evolution
The Solar System formed from the gravitational collapse of a giant molecular
cloud 4.6 billion years ago. This initial cloud was likely several light-years
across and probably birthed several stars. As the region that would become the
Solar System, known as the pre-solar nebula collapsed, conservation of angular
momentum made it rotate faster. The centre, where most of the mass collected,
became increasingly hotter than the surrounding disc. As the contracting nebula
rotated, it began to flatten into a spinning protoplanetary disc with a diameter of
roughly 200 AU and a hot, dense protostar at the centre. At this point in its
evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri
stars show that they are often accompanied by discs of pre-planetary matter with
masses of 0.001–0.1 solar masses, with the vast majority of the mass of the
nebula in the star itself. The planets formed by accretion from this disk.73, 74
Within 50 million years, the pressure and density of hydrogen in the centre of
the protostar became great enough for it to begin thermonuclear fusion. The
temperature, reaction rate, pressure, and density increased until hydrostatic
equilibrium was achieved, with the thermal energy countering the force of
gravitational contraction. At this point the Sun became a full-fledged main
sequence star.
The Solar System as we know it today will last until the Sun begins its
evolution off of the main sequence of the Hertzsprung-Russell diagram. As the
Sun burns through its supply of hydrogen fuel, the energy output supporting the
core tends to decrease, causing it to collapse in on itself. This increase in
pressure heats the core, so it burns even faster. As a result, the Sun is growing
brighter at a rate of roughly ten percent every 1.1 billion years.74, 75


Around 5.4 billion years from now, the hydrogen in the core of the Sun will
have been entirely converted to helium, ending the main sequence phase. At this
time, the outer layers of the Sun will expand to roughly up to 260 times its
current diameter; the Sun will become a red giant. Because of its vastly
increased surface area, the surface of the Sun will be considerably cooler than it
is on the main sequence (2600 K at the coolest).
Eventually, the Sun’s outer layers will fall away, leaving a white dwarf, an
extraordinarily dense object, half the original mass of the Sun but only the size
of the Earth. The ejected outer layers will form what is known as a planetary
nebula, returning some of the material that formed the Sun to the interstellar

The Sun is the star at the center of the Solar System. The Earth and other matter
(including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun,
which by itself accounts for about 99.86% of the Solar System’s mass. The
mean distance of the Sun from the Earth is approximately 149.6 million
kilometers (1 AU), and its light travels this distance in 8 minutes and 19
seconds. This distance varies throughout the year from a minimum of 147.1
million kilometers (0.9833 AU) on the perihelion (around 3 January), to a
maximum of 152.1 million kilometers (1.017 AU) on the aphelion (around 4
July). Energy from the Sun, in the form of sunlight, supports almost all life on
Earth via photosynthesis, and drives the Earth’s climate and weather.
The Sun consists of hydrogen (about 74% of its mass, or 92% of its volume),
helium (about 24% of mass, 7% of volume), and trace quantities of other
elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon,
neon, calcium, and chromium. The Sun has a spectral class of G2V. G2 means
that it has a surface temperature of approximately 5,780 K (5,510°C) giving it a
white color, which often appears as yellow when seen from the surface of the
Earth because of atmospheric scattering. It is this scattering of light at the blue


end of the spectrum that gives the surrounding sky its color. The Sun’s spectrum
contains lines of ionized and neutral metals as well as very weak hydrogen
lines. The V (Roman five) in the spectral class indicates that the Sun, like most
stars, is a main sequence star. This means that it generates its energy by nuclear
fusion of hydrogen nuclei into helium. There are more than 100 million G2
class stars in our galaxy. Once regarded as a small and relatively insignificant
star, the Sun is now presumed to be brighter than 85% of the stars in the galaxy,
most of which are red dwarfs.77, 78
The Sun’s hot corona continuously expands in space creating the solar wind, a
hypersonic stream of charged particles that extends to the heliopause at roughly
100 AU. The bubble in the interstellar medium formed by the solar wind, the
heliosphere, is the largest continuous structure in the Solar System.
The Sun is currently traveling through the Local Interstellar Cloud in the lowdensity Local Bubble zone of diffuse high-temperature gas, in the inner rim of
the Orion Arm of the Milky Way Galaxy, between the larger Perseus and
Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 lightyears (1.6×1014 km) from the Earth, the Sun ranks 4th in mass. The Sun orbits
the center of the Milky Way galaxy at a distance of approximately 24,000–
26,000 light years from the galactic center, moving generally in the direction of
Cygnus and completing one revolution in about 225–250 million years (one
Galactic year). Its orbital speed was thought to be 220 ± 20, km/s but a new
estimate gives 251 km/s. Since our galaxy is moving with respect to the cosmic
microwave background radiation (CMB) in the direction of Hydra with a speed
of 550 km/s, the Sun’s resultant velocity with respect to the CMB is about 370
km/s in the direction of Crater or Leo.79, 80

Motion and Location of Sun within the Galaxy
The Sun lies close to the inner rim of the Milky Way Galaxy’s Orion Arm, in
the Local Fluff or the Gould Belt, at a hypothesized distance of 7.5–8.5 kpc
(25,000–28,000 light years) from the Galactic Center, contained within the


Local Bubble, a space of rarefied hot gas, possibly produced by the supernova
remnant, Geminga. The distance between the local arm and the next arm out,
the Perseus Arm, is about 6,500 light-years. The Sun, and thus the Solar
System, is found in what scientists call the galactic habitable zone.81
The Apex of the Sun’s Way, or the solar apex, is the direction that the Sun
travels through space in the Milky Way. The general direction of the Sun’s
galactic motion is towards the star Vega near the constellation of Hercules, at an
angle of roughly 60 sky degrees to the direction of the Galactic Center. If one
were to observe it from Alpha Centauri, the closest star system, the Sun would
appear to be in the constellation Cassiopeia. The Sun’s orbit around the Galaxy
is expected to be roughly elliptical with the addition of perturbations due to the
galactic spiral arms and non-uniform mass distributions. In addition the Sun
oscillates up and down relative to the galactic plane approximately 2.7 times per
orbit. This is very similar to how a simple harmonic oscillator works with no
drag force (damping) term.82, 83
It has been argued that the Sun’s passage through the higher density spiral arms
often coincides with mass extinctions on Earth, perhaps due to increased impact
events. It takes the Solar System about 225–250 million years to complete one
orbit of the galaxy (a galactic year), so it is thought to have completed 20–25
orbits during the lifetime of the Sun. The orbital speed of the Solar System
about the center of the Galaxy is approximately 251 km/s. At this speed, it takes
around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8
days to travel 1 AU. The Sun’s motion about the centre of mass of the Solar
System is complicated by perturbations from the planets. Every few hundred
years this motion switches between prograde and retrograde.84, 85

Characteristics of Sun
The Sun is a yellow main sequence star comprising about 99.86% of the total
mass of the Solar System. It is a near-perfect sphere, with an oblateness
estimated at about 9 millionths, which means that its polar diameter differs from


its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic
state and is not solid, it rotates faster at its equator than at its poles. This
behavior is known as differential rotation. The period of this actual rotation is
approximately 25.6 days at the equator and 33.5 days at the poles. However, due
to our constantly changing vantage point from the Earth as it orbits the Sun, the
apparent rotation of the star at its equator is about 28 days. The centrifugal
effect of this slow rotation is 18 million times weaker than the surface gravity at
the Sun’s equator. The tidal effect of the planets is even weaker, and does not
significantly affect the shape of the Sun.86
The Sun is a Population I, or heavy element-rich star. The formation of the Sun
may have been triggered by shockwaves from one or more nearby supernovae.
This is suggested by a high abundance of heavy elements in the Solar System,
such as gold and uranium, relative to the abundances of these elements in socalled Population II (heavy element-poor) stars. These elements could most
plausibly have been produced by endergonic nuclear reactions during a
supernova, or by transmutation via neutron absorption inside a massive secondgeneration star. The Sun does not have a definite boundary as rocky planets do,
and in its outer parts the density of its gases drops approximately exponentially
with increasing distance from its center. Nevertheless, it has a well-defined
interior structure. The Sun’s radius is measured from its center to the edge of the
photosphere. This is simply the layer above which the gases are too cool or too
thin to radiate a significant amount of light, and is therefore the surface most
readily visible to the naked eye.86
The solar interior is not directly observable, and the Sun itself is opaque to
electromagnetic radiation. However, just as seismology uses waves generated
by earthquakes to reveal the interior structure of the Earth, the discipline of
helioseismology makes use of pressure waves (infrasound) traversing the Sun’s
interior to measure and visualize the star’s inner structure. Computer modeling
of the Sun is also used as a theoretical tool to investigate its deeper layers.87


The core of the Sun is considered to extend from the center to about 0.2 to 0.25
solar radii. It has a density of up to 150 g/cm3 (150 times the density of water on
Earth) and a temperature of close to 13,600,000 Kelvin (by contrast, the surface
of the Sun is around 5,800 Kelvin). Recent analysis of SOHO mission data
favors a faster rotation rate in the core than in the rest of the radiative zone.
Through most of the Sun’s life, energy is produced by nuclear fusion through a
series of steps called the p–p (proton–proton) chain; this process converts
hydrogen into helium. Less than 2% of the helium generated in the Sun comes
from the CNO cycle. The core is the only location in the Sun that produces an
appreciable amount of heat via fusion: the rest of the star is heated by energy
that is transferred outward from the core. All of the energy produced by fusion
in the core must travel through many successive layers to the solar photosphere
before it escapes into space as sunlight or kinetic energy of particles.88, 89
About 9.2 × 1037 protons (hydrogen nuclei) are converted into helium nuclei
every second (out of ~8.9 × 1056 total amount of free protons in the Sun), or
about 4.4 × 109 kg per second, releasing energy at the matter–energy conversion
rate of 4.26 million metric tons per second, 383 yottawatts (3.83×1026 W), or
9.15 × 1010 megatons of TNT per second. Power density is about 194 µW/kg of
matter, though since most fusion occurs in the relatively small core the plasma
power density there is about 150 times bigger. For comparison, the human body
produces heat at approximately the rate 1.3 W/kg, roughly 600 times greater per
unit mass. Assuming core density 150 times higher than average, this
corresponds to a surprisingly low rate of energy production in the Sun’s core—
about 0.272 W/m3. This power is much less than generated by a single candle.
The use of plasma with similar parameters for energy production on Earth
would be completely impractical—even a modest 1 GW fusion power plant
would require about 5 billion metric tons of plasma.89, 90
The rate of nuclear fusion depends strongly on density and temperature, so the
fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of


fusion would cause the core to heat up more and expand slightly against the
weight of the outer layers, reducing the fusion rate and correcting the
perturbation; and a slightly lower rate would cause the core to cool and shrink
slightly, increasing the fusion rate and again reverting it to its present level.91
The high-energy photons (gamma rays) released in fusion reactions are
absorbed in only a few millimeters of solar plasma and then re-emitted again in
random direction (and at slightly lower energy)—so it takes a long time for
radiation to reach the Sun’s surface. Estimates of the photon travel time range
between 10,000 and 170,000 years. After a final trip through the convective
outer layer to the transparent surface of the photosphere, the photons escape as
visible light. Each gamma ray in the Sun’s core is converted into several million
visible light photons before escaping into space. Neutrinos are also released by
the fusion reactions in the core, but unlike photons they rarely interact with
matter, so almost all are able to escape the Sun immediately.92, 93

Radiative Zone
From about 0.25 to about 0.7 solar radii, solar material is hot and dense enough
that thermal radiation is sufficient to transfer the intense heat of the core
outward. In this zone there is no thermal convection; while the material grows
cooler as altitude increases (from 7,000,000°C to about 2,000,000°C) this
temperature gradient is less than the value of adiabatic lapse rate and hence
cannot drive convection. Heat is transferred by radiation—ions of hydrogen and
helium emit photons, which travel only a brief distance before being reabsorbed
by other ions. The density drops a hundredfold (from 20 g/cm³ to only 0.2
g/cm³) from the bottom to the top of the radiative zone.94
Between the radiative zone and the convection zone is a transition layer called
the tachocline. This is a region where the sharp regime change between the
uniform rotation of the radiative zone and the differential rotation of the
convection zone results in a large shear—a condition where successive
horizontal layers slide past one another. The fluid motions found in the


convection zone above, slowly disappear from the top of this layer to its bottom,
matching the calm characteristics of the radiative zone on the bottom. Presently,
it is hypothesized that a magnetic dynamo within this layer generates the Sun’s
magnetic field.95

Convective Zone
In the Sun’s outer layer, from its surface down to approximately 200,000 km (or
70% of the solar radius), the solar plasma is not dense enough or hot enough to
transfer the heat energy of the interior outward via radiation (in other words it is
opaque enough). As a result, thermal convection occurs as thermal columns
carry hot material to the surface (photosphere) of the Sun. Once the material
cools off at the surface, it plunges back downward to the base of the convection
zone, to receive more heat from the top of the radiative zone. At the visible
surface of the Sun, the temperature has dropped to 5,700°K and the density to
only 0.2 g/m³ (about 1/10,000th the density of air at sea level).95, 96
The thermal columns in the convection zone form an imprint on the surface of
the Sun, in the form of the solar granulation and supergranulation. The turbulent
convection of this outer part of the solar interior gives rise to a “small-scale”
dynamo that produces magnetic north and south poles all over the surface of the
Sun. The Sun’s thermal columns are Bénard cells and therefore tend to be
hexagonal prisms.

The visible surface of the Sun, the photosphere, is the layer below which the
Sun becomes opaque to visible light. Above the photosphere visible sunlight is
free to propagate into space, and its energy escapes the Sun entirely. The change
in opacity is due to the decreasing amount of− Hions, w

hich absorb visible

light easily. Conversely, the visible light we see is produced as electrons react
with hydrogen atoms to produce H
− ions. The photosphere is actually tens to
hundreds of kilometers thick, being slightly less opaque than air on Earth.


Because the upper part of the photosphere is cooler than the lower part, an
image of the Sun appears brighter in the center than on the edge or limb of the
solar disk, in a phenomenon known as limb darkening. Sunlight has
approximately a black-body spectrum that indicates its temperature is about
6,000 K, interspersed with atomic absorption lines from the tenuous layers
above the photosphere. The photosphere has a particle density of ~1023 m−3 (this
is about 1% of the particle density of Earth’s atmosphere at sea level).97, 98
During early studies of the optical spectrum of the photosphere, some
absorption lines were found that did not correspond to any chemical elements
then known on Earth. In 1868, Norman Lockyer hypothesized that these
absorption lines were because of a new element which he dubbed “helium”,
after the Greek Sun god Helios. It was not until 25 years later that helium was
isolated on Earth.99

The parts of the Sun above the photosphere are referred to collectively as the
solar atmosphere. They can be viewed with telescopes operating across the
electromagnetic spectrum, from radio through visible light to gamma rays, and
comprise five principal zones: the temperature minimum, the chromosphere, the
transition region, the corona, and the heliosphere. The heliosphere, which may
be considered the tenuous outer atmosphere of the Sun, extends outward past
the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary
with the interstellar medium. The chromosphere, transition region, and corona
are much hotter than the surface of the Sun. The reason why has not been
conclusively proven; evidence suggests that Alfvén waves may have enough
energy to heat the corona.100
The coolest layer of the Sun is a temperature minimum region about 500 km
above the photosphere, with a temperature of about 4,100 K. This part of the
Sun is cool enough to support simple molecules such as carbon monoxide and
water, which can be detected by their absorption spectra.


Above the temperature minimum layer is a layer about 2,000 km thick,
dominated by a spectrum of emission and absorption lines. It is called the
chromosphere from the Greek root chroma, meaning color, because the
chromosphere is visible as a colored flash at the beginning and end of total
eclipses of the Sun. The temperature in the chromosphere increases gradually
with altitude, ranging up to around 20,000 K near the top. In the upper part of
chromosphere helium becomes partially ionized.
Above the chromosphere there is a thin (about 200 km) transition region in
which the temperature rises rapidly from around 20,000 K in the upper
chromosphere to coronal temperatures closer to one million K. The temperature
increase is facilitated by the full ionization of helium in the transition region,
which significantly reduces radiative cooling of the plasma. The transition
region does not occur at a well-defined altitude. Rather, it forms a kind of
nimbus around chromospheric features such as spicules and filaments, and is in
constant, chaotic motion. The transition region is not easily visible from Earth’s
surface, but is readily observable from space by instruments sensitive to the
extreme ultraviolet portion of the spectrum.97, 101
The corona is the extended outer atmosphere of the Sun, which is much larger in
volume than the Sun itself. The corona continuously expands into the space
forming the solar wind, which fills all Solar System. The low corona, which is
very near the surface of the Sun, has a particle density around 1015–1016 m−3.
The average temperature of the corona and solar wind is about 1–2 million
kelvins, however, in the hottest regions it is 8–20 million kelvins. While no
complete theory yet exists to account for the temperature of the corona, at least
some of its heat is known to be from magnetic reconnection.
The heliosphere, which is the cavity around the Sun filled with the solar wind
plasma, extends from approximately 20 solar radii (0.1 AU) to the outer fringes
of the Solar System. Its inner boundary is defined as the layer in which the flow
of the solar wind becomes superalfvénic—that is, where the flow becomes


faster than the speed of Alfvén waves. Turbulence and dynamic forces outside
this boundary cannot affect the shape of the solar corona within, because the
information can only travel at the speed of Alfvén waves. The solar wind travels
outward continuously through the heliosphere, forming the solar magnetic field
into a spiral shape, until it impacts the heliopause more than 50 AU from the
Sun. In December 2004, the Voyager 1 probe passed through a shock front that
is thought to be part of the heliopause. Both of the Voyager probes have
recorded higher levels of energetic particles as they approach the boundary.102104

Magnetic field of Sun
The Sun is a magnetically active star. It supports a strong, changing magnetic
field that varies year-to-year and reverses direction about every eleven years
around solar maximum. The Sun’s magnetic field gives rise to many effects that
are collectively called solar activity, including sunspots on the surface of the
Sun, solar flares, and variations in solar wind that carry material through the
Solar System. Effects of solar activity on Earth include auroras at moderate to
high latitudes, and the disruption of radio communications and electric power.
Solar activity is thought to have played a large role in the formation and
evolution of the Solar System. Solar activity changes the structure of Earth’s
outer atmosphere.104, 105
All matter in the Sun is in the form of gas and plasma because of its high
temperatures. This makes it possible for the Sun to rotate faster at its equator
(about 25 days) than it does at higher latitudes (about 35 days near its poles).
The differential rotation of the Sun’s latitudes causes its magnetic field lines to
become twisted together over time, causing magnetic field loops to erupt from
the Sun’s surface and trigger the formation of the Sun’s dramatic sunspots and
solar prominences. This twisting action gives rise to the solar dynamo and an
11-year solar cycle of magnetic activity as the Sun’s magnetic field reverses
itself about every 11 years.106


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